This invention relates to a means and method for the reduction of hoop stress in silos which contain grain, or other bulk solids.
Many silos, which were built before the dynamics of discharge, were fully understood and designed for static loading, but it has been shown that material pressures exerted on the cell walls increases by a factor of up to about two and a half when the outloading valve is opened and material begins to move.
The opening of the valve removes vertical support from the material directly about it and the stress field changes from "peaked", with lines of major stress vertical, or near vertical, to "arched" with lines of major stress arching across the cell.
The "arched" stress field occupies a conical zone which diverges upwardly. At the point where this conical zone intersects the cell walls, the large lateral component of force causes a high hoop stress in the cell walls. This stress exceeds the static stress by a factor of up to two and a half, and is often large enough to cause overstressing and cracking of cell walls.
The cost of replacement of a silo is prohibitive, and the main object of this invention is to provide improvements whereby the hoop stress can be substantially reduced.
Several methods are available to strengthen the cylindrical walls of an upstanding silo. One widely used (but basically unsound) method, is the repair of bulged areas, but even this is expensive. The second alternative is the use of external strapping on the external surfaces of external cells only of a group of silos, but this is many times more expensive than the cost of local bulge repairs. Another possibility which has been examined has been the use of a steel liner spirally wound within a silo to lie against the inner surface of a concrete wall, but this is even more expensive than the external strapping. The other alternative (apart from this invention) is the use of a concrete liner constructed for the full height of the cell and within an old cell, but the cost of this is so great that it is not viable.
The object of this invention is to provide improvements which are economical and feasible. STATE OF THE ART
This problem has already been the subject of various studies and the following references are pertinent:
(a) Arnold, P. C., McLean, A. G. and Roberts, A. W. BULK SOLIDS: STORAGE, FLOW AND HANDLING. Tunra Bulk Solids Handling Research Associates. PA1 (b) Jenike, A. S. GRAVITY FLOW OF BULK SOLIDS, BULLETIN 108, Utah Engineering Experiment Station, University of Utah. PA1 (c) Riembert, M. & A. SILOS, THEORY AND PRACTICE, Trans Tech Publications, 1976. PA1 (d) Warner, R. F. STRENGTHENING STIFFENING AND REPAIR OF CONCRETE STRUCTURES, IABSE SURVEYS 17/81. PA1 (e) Reimbert, A. U. S. Pat. No. 4,372,466.
The reader's attention is drawn to a central tube known as an "anti-dynamic tube" proposed by Riembert, and this employs a tube containing a plurality of apertures throughout its length, placed at the cell center and extending for full cell height, and supported by guy wires fixed to the cell wall. In principle, the tube and portholes are intended to ensure that the grain flows into the tube only close to the grain surface, thus emptying the cell from the top downwards. No mass flow occurs, and no switch pressures are generated against the cell walls. Although there is available supporting literature, there appears to be some practical problems. The small portholes are liable to blockage, thus causing unsymmetrical flow, which in turn generates large lateral forces on the tube with the possibility of collapse. If flow into the tube is able to occur at lower tube levels than close to the free grain surface, there will be mass flow within the bin.
The Reimbert U. S. Pat. No. 4,372,466 also discloses use of a central tube (5) which had imperforate walls, and was separately valved from the rest of the silos. Although this arrangement is capable of effective use, it is also capable of incorrect use, and if for example, the second discharge orifice (4) is opened before the first discharge orifice (3), the arrangement is ineffectual, and high stresses can be imparted to the silo walls.
Many silo cells have a height to diameter ratio of about three, and the hoop stress is excessive only when the ratio exceeds about 1.5 (depending upon the grain used and its moisture content), and this invention seeks to resolve the excessive hoop stress by dividing a silo cell into a plurality of notional cells one about the other.